Jeff would likely pick up that there doesn't actually appear to be anything directly to make contact with. In fact, it's more like there's an echo of something that was there, but has now disappeared. Like he's trying to interact with an afterimage. He'd also be among the first, probably, to pick up that there is a definitive source for the effect at the circus grounds and that the effect diminishes the farther away from them you get. He's also likely to experience more severe and prolonged hallucinations even as the effects diminish for others. They will eventually resolve, but staring into that is going to extend the duration.

Nothing would show up on a mirror selfie except some peculiar blurring. Clean your lens, Jeff. Maybe it's just all in your head~.